1. Field of the Invention
The present invention relates generally to an oxygen sensor for determining an oxygen concentration in exhaust gases such as those emitted by internal combustion engines and boilers, and a method of producing such an oxygen sensor. More particularly, the invention is concerned with a technique useful for improving the durability of such an oxygen sensor.
2. Discussion of the Prior Art
In the art of controlling an air/fuel (A/F) ratio or burning condition of an air-fuel mixture supplied to automotive internal combustion engines, boilers or other equipment, it is known to detect the oxygen concentration of exhaust gases emitted by such engines or boilers, by utilizing a sensor which uses an oxygen-ion conductive solid electrolyte such as zirconia ceramics. The sensor is operated according to the principle of an oxygen concentration cell.
Such an oxygen sensor for detecting the oxygen concentration employs a sensing element which includes a tubular solid electrolyte body which is closed at one of its opposite ends and open at the other end. The solid electrolyte body has an inner and an outer electrode formed on its respective inner and outer surfaces. The inner electrode serves as a reference electrode which is exposed to ambient air as a reference gas having a known oxygen concentration. On the other hand, the outer electrode serves as a measuring electrode which is exposed to a measurement gas in the form of the exhaust gases emitted by an internal combustion engine. According to this oxygen sensor, the concentration of oxygen in the exhaust gases is determined by measuring an electromotive force that is induced between the reference and measuring electrodes, based on a difference in the oxygen concentration between the reference gas and the measurement gas.
In a known oxygen sensor of the type described above, an oxygen-ion conductive solid electrolyte constitutes a suitably shaped main body of the oxygen sensing element, on which the electrodes are formed in contact with the surfaces of the solid electrolyte. In operation, the measuring electrode formed on the outer surface of the solid electrolyte main body is subject to heat of the exhaust gases having a generally high temperature. Consequently the measuring electrode tends to suffer from thermal wear or damage, and the sensing capability or measuring accuracy of the oxygen sensor is deteriorated. To solve this problem, it has been proposed to protect the measuring electrode from direct exposure to the exhaust gases by covering the measuring electrode with a porous protective coating having a suitable thickness, which is formed by plasma-spraying spinel or other ceramic material over the measuring electrode.
However, the protective coating formed on the sensing element of the oxygen sensor suffers from separation or peel-off from the surface of the solid electrolyte main body, due to repetitive thermal expansion and contraction caused by an excessively large change in the environmental temperature of the sensor during its use while being attached to the exhaust pipe of an automotive engine, for example. To alleviate ths problem, it has been proposed to increase the adhesive strength of the protective coating relative to the solid electrolyte main body. This requires an increased amount of energy to effect a plasma-spraying of the appropriate ceramic material to the main body. In this case, however, the protective coating tends to be too dense to permit easy permeation of the measurement gas therethrough, leading to an extremely low operating response of the sensing element. Further, such a relatively dense protective coating may be readily clogged by fine particles of iron, phosphorus, zinc or other substances contained in the measurement gas. This also leads to the deterioration of the operating response of the oxygen sensor.
On the other hand, there has been proposed another technique to improve the adhesion between the measuring electrode and the solid electrolyte main body. An example of this technique is disclosed in U.S. Pat. No. 4,477,487 and German Patent No. 3118299, wherein the measuring electrode is disposed on an undulated outer layer of a solid electrolyte formed as an integral outer part of the main body of the sensing element. The adhesive strength between the electrode and the main body is increased owing to an increased area of contact therebetween, due to the engagement of the electrode with the relatively short-pitched convexed and concaved portions of the undulated outer layer of the main body. To further improve the durability of the measuring electrode thus formed on the undulated surface portions of the main body, it is attempted to utilize the above-indicated technique to apply a porous ceramic protective coating to the measuring electrode, to protect the measuring electrode.
In the oxygen sensing element with its measuring electrode formed on its undulated outer surface and covered by the protective coating, as described above, the measuring electrode still suffers from a problem, that is, deterioration, evaporation or expansion due to its exposure to high-temperature exhaust gases. This thermal problem may often cause peel-off or separation of the protective coating due to an expanding force of the electrode acting on the protective coating. Thus, the above-indicated known sensing element is not completely satisfactory, in terms of the durability of the protective coating.